1. Field of the Invention
The present invention relates to an apparatus which processes a substrate that includes a mark, and a method of obtaining a position of a mark included in a substrate.
2. Description of the Related Art
Japanese Patent Laid-open No. 11-054418 discloses an alignment method in an exposure apparatus, and more particularly an alignment method for correcting a signal waveform obtained from a surface to be observed (a wafer mark). In addition, Japanese Patent Laid-open No. 11-054418 discloses a method and an apparatus of correcting an illuminance unevenness, a sensitivity unevenness such as a light receiving device or an image pickup device, or a distortion caused by the adhesion of dust or the like, for the signal waveform obtained by illuminating and observing the surface to be observed which is used for an accurate position measurement or the like.
In a previous exposure apparatus, an image pickup device which mounts an electronic shutter was used. Therefore, an illumination time period of illumination light was able to be accurately controlled. However, recently, an image pickup device having a large number of pixels in order to improve the accuracy may be used. The image pickup device having the large number of pixels cannot mount any electronic shutter. In this case, it is necessary to control the illumination time period of the illumination light using a mechanical opening and closing device such as a mechanical shutter.
The mechanical shutter cannot open and close instantly, which is different from the electronic shutter. Therefore, a predetermined driving time period until the mechanical shutter is fully open is necessary, and also the light intensity distribution on the image pickup device is nonuniform until the mechanical shutter is fully open.
FIGS. 4A to 4C are diagrams showing changes of illuminance unevennesses when the illumination time period of the illumination light is changed. FIG. 4A shows a case where the illumination time period is short (a case where a reflectance of the wafer is high), and FIG. 4B shows a case where the illumination time period is long (a case where the reflectance of the wafer is low). FIG. 4C shows illuminance unevennesses A and B in cases of FIGS. 4A and 4B, respectively.
As shown in FIGS. 4A and 4B, when the illumination time period of the illumination light is changed, a ratio of the illumination time period of the illumination light and the driving time period of the mechanical shutter (the shutter) changes. Therefore, the influence of the illuminance unevenness during driving the mechanical shutter varies. Accordingly, as shown in FIG. 4C, the illuminance unevenness in the entire illumination time period of the illumination light also varies. As a result, a signal waveform correcting method using a fixed correction formula for the illumination time period of the illumination light like a previous method, it is difficult to accurately correct the signal waveform.